1. Field of the Invention
This invention relates generally to the art of forming holes with a laser beam and more particularly to a method of and apparatus for utilizing a laser to perforate an elongate member, such as a tube, at intervals along the member.
2. Discussion of the Prior Art
As will become apparent from the ensuing description, the laser beam perforating method and apparatus of the invention may be utilized to perforate a variety of elongate members for various purposes. The primary application of the invention, however, involves perforating at intervals a thin walled flexible tube of thermoplastic material to form a so-called drip irrigation tube. Such a drip irrigation tube is a hose-like plastic tube which is laid across the ground to be irrigated and connected at one end to a source of water under pressure. Extending through the tube wall at intervals along the tube are small drip holes through which the water emerges in the form of droplets to irrigate the ground. The invention will be described in the context of this primary application.
Some drip irrigation tubes contain a single water passage. In this type of tube, all of the holes in the tube wall are drip holes which open to the passage. Other drip irrigation tubes contain a primary water passage bounded by a first tube wall and a secondary water passage bounded by a portion of the first wall and a second outer wall of the tube whereby the first wall portion forms a separation wall between the tube passages. The primary passage communicates directly at one end to the source of water under pressure. The secondary water passage extends the full length of the primary passage and communicates to the latter passage through holes located at intervals along the tube passage separation wall. The second outer tube wall contains spaced drip holes communicating to the secondary water passage through which water drips from the latter passage to the ground.
A variety of techniques have been devised for perforating drip irrigation tubes of the kind described above and other elongate members. Prior art of interest in this connection are U.S. Pat. Nos. 3,594,261, 3,626,143 and 3,808,394. Of particular interest in connection with perforating drip irrigation tubes is the above mentioned U.S. Pat. No. 3,808,394 which utilizes a laser beam to form the tube perforations or water drip holes. The laser beam perforating apparatus of this latter patent, however, has certain deficiencies which detract from its usefulness and which need not be elaborated on in detail in this disclosure. Suffice it to say that this patented apparatus is quite costly and complex and, most important, permits perforation of a tube at one fixed spacing only. The other laser perforating devices and techniques are also deficient in one way or another, and hence there is a need for an improved laser perforating method and apparatus, particularly for perforating drip irrigation tubes of the class described.